Gemini surfactants at the air/water interface: a fully atomistic molecular dynamics study

Area of Science:

• Physical Chemistry
• Surface Science
• Computational Chemistry

Background:

• Gemini surfactants, with two chains linked by a spacer, exhibit unique interfacial properties.
• Understanding their behavior at interfaces is crucial for applications in various fields.

Purpose of the Study:

• To investigate the interfacial behavior of Gemini surfactants using molecular dynamics (MD) simulations.
• To determine the conditions for stable monolayer formation at the air/water interface.
• To elucidate the influence of spacer length and hydrophobicity on surfactant structure and aggregation.

Main Methods:

• Fully atomistic molecular dynamics (MD) simulations were performed for a series of Gemini surfactants with varying spacer lengths (s = 3-16).
• Simulations were conducted at experimentally determined surface areas per surfactant (AS) at the critical micelle concentration (cmc) and one-tenth of cmc.
• Density profiles were analyzed and compared with neutron reflection experimental data.

Main Results:

• Stable monolayers at the air/water interface were observed only for Gemini surfactants with s = 12 and 14, and for s = 3 at a lower surface area.
• Surfactants with shorter spacers or larger surface areas showed partial desorption into the aqueous phase, forming aggregates.
• MD simulations confirmed spacer bending towards hydrophobic chains with increasing spacer length and hydrophobicity.

Conclusions:

• The stability of Gemini surfactant monolayers is highly dependent on spacer length and surface area.
• Molecular dynamics simulations provide valuable insights into the interfacial behavior of Gemini surfactants, complementing experimental observations.
• The findings contribute to a deeper understanding of surfactant self-assembly and interfacial phenomena.